Fragile X syndrome (FXS) is the leading single gene cause of intellectual disability and autism. Study of FXS thus provides insights into conditions that affect large populations of Americans. FXS is caused by mutations in the gene encoding the RNA binding protein FMRP. In the absence of FMRP, synaptic plasticity is altered. Both intellectual disability and autism likely result from impairments in a restricted set of neural circuits. A large body of work has shown that FMRP is expressed in the somatodendritic domain of virtually every neuron. However, we have recently described a novel structure, the Fragile X granule (FXG), that is expressed presynaptically in a subset of circuits, including some implicated in the etiology of autism such as frontal cortex, cerebellum and amygdala. Importantly, FXG expression is developmentally regulated. Our results suggest that some of the symptoms of FXS and autism may arise from perturbations in presynaptic FMRP. In my proposed work, I will extend and deepen our understanding of presynaptic FMRP and FXGs by addressing three goals: 1) To characterize the expression and activity-dependent regulation of presynaptic FMRP in vivo;2) To identify FXG-associated RNAs;3) To determine domains of FMRP required for FXG formation and presynaptic differentiation. Findings from these studies will elucidate how presynaptic FMRP contributes to normal cognition and how its loss contributes to the phenotypes observed in FXS and potentially autistic patients. My career objective is to become an independent researcher at an academic institution where I can combine teaching and research. My long-term research interests are on how the environment shapes the structure and ultimately the function of the nervous system. My research as a doctoral student at Yale University studying Neuroscience focused on the anatomy of the olfactory system and how its structure can be used to understand its function. For my postdoctoral studies, I have been broadening my technical expertise to include molecular biological, cell biological, and biochemical approaches. The acquisition of this expertise will be crucial to the establishment of my independent lab. I am working with Dr. Justin Fallon at Brown University due to his well-established expertise with these approaches as well as his scientific interest in the structural plasticity of synapses in response to interactions with the environment. The technical and intellectual environments of his lab are thus ideal for my continued progress. The proposed K99 phase of the research will involve learning new techniques both from Dr. Fallon and from collaborations with faculty at Brown and outside institutions. Prof. Fallon has been very supportive in the establishment of these collaborations. My progress through this phase will be monitored by Dr. Fallon and will be supported by an advisory committee comprising Drs. Gilad Barnea, Julie Kauer, and Kim Mowry. Their support, along with the broader institutional support including the outstanding research facilities required for the conduct of my research, will greatly facilitate the execution of my planned experiments and my subsequent transition to independence. PUBLIC HEALTH RELEVANCE: Fragile X syndrome (FXS), the leading single gene cause of both mental retardation and autism, arises from mutations in the gene encoding the protein FMRP. Our recent work has indicated that some of the deficits seen in FXS may arise from loss of FMRP function at presynaptic sites. The proposed work will address this possibility and may lead to treatments for FXS and autism.